Will be replaced in your husband's life by another woman hurts a lot. You are likely to cycle through periods of anger and sadness and the sickening sense of how you were betrayed. Before you throw in the towel on your marriage, take a closer look at whether your marriage can still be saved. The investment it takes to do it is worth it to be able to clear your mind and breathe easy again.
Men and women handle their thinking and their feelings in different ways. In general, men are much better to separate their work life from their family life, their sex from their romance and their girlfriend from their wives. They may have all of these mental "boxes" and mentally jump from one to the other during the day. Out of sight is out of mind. As long as he can keep all its boxes separated, there is no problem as far as he is concerned.
Some men can have a lover and a married life for years because of their ability to divide. This is especially true if the other woman will play along with confidentiality. The other woman, though, are usually not money-grubbing, unkind, so and so is what you want to believe that she is. She is often under the assumption that your husband is not only suffering in her marriage, but that he plans to leave all of you.